1. Field of the Invention
The invention generally relates to balancing electrical current in loads with a negative impedance characteristic. In particular, the invention relates to balancing electrical current used in driving multiple gas discharge tubes, such as multiple cold cathode fluorescent lamps (CCFLs).
2. Description of the Related Art
Cold cathode fluorescent lamps (CCFLs) are used in a broad variety of applications as light sources. For example, CCFLs can be found in lamps, in scanners, in backlights for displays, such as liquid crystal displays (LCDs), and the like. In recent years, the size of LCD displays has grown to relatively large proportions. Relatively large LCDs are relatively common in computer monitor applications, in flat-screen televisions, and in high-definition televisions. In these and many other applications, the use of multiple CCFLs is common. For example, a combination of six CCFLs is relatively common in a backlight for a desktop LCD computer monitor. In another example of a relatively-large flat-screen television, 16, 20, 32, and 40 CCFLs have been used. Of course, the number of CCFLs used in any particular application can vary in a very broad range.
Desirably, in applications with multiple CCFLs, the CCFLs are driven by relatively few power inverters to save size, weight, and cost. However, driving multiple CCFLs from a single or relatively few power inverters is a relatively difficult task. When multiple CCFLs are coupled in series, the operating voltage required to light the series-coupled lamps increases to impractical levels. The increase in operating voltage leads to increased corona discharge, requires expensive high voltage insulation, and the like.
Coupling CCFLs in parallel provides other problems. While the operating voltage of paralleled lamps is desirably low, relatively even current balancing in paralleled CCFLs can be difficult to achieve in practice. CCFLs and other gas discharge tubes exhibit a negative impedance characteristic in that the hotter and brighter a particular CCFL tube runs, the lower its impedance characteristic and the higher its drawn current. As a result, when CCFLs are paralleled without balancing circuits, some lamps will typically be much brighter than other lamps. In many cases, some lamps will be on, while other lamps will be off. In addition to the drawbacks of uneven illumination, the relatively brighter lamps can overheat and exhibit a short life.
A two-way balancing transformer can be used to balance current in two CCFLs. This type of balancing transformer can be constructed from two relatively equal windings on the same core and is sometimes referred to in the art as a “balun” transformer, though it will be understood that the term “balun” applies to other types of transformers as well. While the two-way balancing transformer technique works well to balance current when both CCFLs are operating, when one of the two CCFLs fails, the differential voltage across the two-way balancing transformer can grow to very high levels. This differential voltage can damage conventional two-way balancing transformers. In addition, conventional configurations with two-way balancing transformers are limited to paralleling two CCFLs. Another drawback of conventional balancing transformer configurations is relatively inefficient suppression of electromagnetic interference (EMI).